A superabsorbent polymer (SAP) is a synthetic polymeric material capable of absorbing moisture from about 500 to 1000 times its own weight. Various manufacturers have given it different names, such as a SAM (Super Absorbency Material), an AGM (Absorbent Gel Material), etc. Such superabsorbent polymers started to be practically applied in sanitary products, and now they are widely used not only for hygiene products such as disposable diapers for children, etc., but also for water retaining soil products for gardening, water stop materials for civil engineering and construction, sheets for raising seedling, fresh-keeping agents for food distribution fields, materials for poultices, etc.
In most cases, these superabsorbent polymers have been widely used in the field of hygienic materials such as diapers, sanitary napkins, etc. For these applications, superabsorbent polymers are required to exhibit a high absorption rate with respect to water, etc., must not release absorbed water even under external pressure, and also must maintain their shape under volume expansion (swelling) after water absorption to show excellent permeability.
Recently, as efforts have been made to provide diapers which are thinner and lighter while showing excellent performance, much attention is focused on superabsorbent polymers having more improved liquid permeability and absorption rate. In order to achieve such high absorption rate and improved liquid permeability, a superabsorbent polymer particle, particularly, a surface-crosslinked layer, must have higher surface strength to show high gel strength, which allows urine to be uniformly and rapidly distributed into an absorbent core of a diaper.
However, in the case of increasing gel strength and improving liquid permeability and absorption rate by using the previously known methods, there is a disadvantage that basic absorption performance (absorption amount under no load and under a load) is greatly reduced.
Accordingly, there is a continuous demand for technologies capable of providing superabsorbent polymers having more improved liquid permeability and absorption rate while maintaining excellent basic absorption performance.